This invention relates to a novel viewing-screen structure for a cathode-ray tube and to a novel method for preparing that viewing-screen structure.
One type of cathode-ray tube that is used for television displays is referred to as a shadow-mask tube. This tube is comprised of an evacuated envelope having a viewing window, a viewing-screen structure comprised of a mosaic of phosphor areas (usually dots or strips) of different emission colors supported on the inner surface of the viewing window, a shadow mask having an array of apertures therein in register with the phosphor areas mounted in the tube in adjacent spaced relation with the window, and means for projecting one or more (usually three) electron beams towards the screen for selectively exciting the phosphor areas of the mosaic.
In operating a shadow-mask tube, the electron beams are made to scan a raster in a fixed pattern. As the beams are made to scan, they are either intercepted by the mask or they pass through the mask apertures and excite the desired phosphor areas. The energy in the intercepted electron beams heats the mask and may cause the mask to become distorted, which may adversely affect the position of the beams which pass through the mask apertures. Some of the heat in the mask is removed by radiation back to a black coating on the funnel of the tube. Normally, the viewing-screen structure includes a thin layer of a highly reflective metal, usually aluminum, which reflects heat that is radiated forward towards the screen.
U.S. Pat. No. 3,703,401, issued Nov. 21, 1972, to Samuel B. Deal and Donald W. Bartch, suggests applying to the reflective metal layer a heat-absorptive overcoating of carbon particles. Then, the structure is baked to remove organic and volatile materials therefrom. The purpose of a heat-absorptive overcoating is to promote the transport of heat from the shadow mask to the atmosphere through the glass panel and thereby reduce mask warpage due to uneven heating of the mask-frame assembly of the tube. Common formulations used in applying these overcoatings include such constituents as finely-divided particles of graphite and lamp black together with dispersants and wetting agents.
Overcoatings produced with the common formulations have been found to be prone to the formation of defects on the viewing screen and on the sidewalls of the screen support during tube fabrication, particularly when exposed to normal humid atmosphere for any extended period of time after the screen is baked. Some of the defects appear as a complete loss of reflectivity in small areas more or less scattered over the reflective layer and sidewalls. The size and frequency of the defects increase with increasing humidity and also with temperature. This humidity or moisture susceptibility of a baked screen structure is a serious problem which may result in costly losses to normal factory processing.